Heretofore, electromagnetic valves have been widely used for controlling a flow rate. Such electromagnetic valve is arranged so that the stroke of a valve element is controlled by balance between an attractive force of a fixed core and a restoring force of a spring. One of those flow control valves is shown in FIG. 27 for example (Patent Document 1).
This flow control valve 210 has a body 232 formed with an inlet passage 234 and outlet passage 235, and a valve chamber 233 which provides communication between the inlet passage 234 and the outlet passage 235. Further, in the valve chamber 233, a valve seat 236 is formed at the end of the outlet passage 235.
Above the body 232, a coil 222 is provided to excite a fixed core 223. The fixed core 223 is placed in an upper part of the coil 222 and a movable core 224 is slidably fitted in the coil 222. An end of the movable core 224 is provided with a valve element 231 and attached with a spring 225. This spring 225 urges the movable core 224 downward.
When the fluid flowing in the flow control valve 210 through the inlet passage 234 is allowed to flow from the inlet passage 234 to the outlet passage 235, the coil 222 is energized to excite the fixed core 223. The excited fixed core 223 attracts the movable core 224 to move against the urging force of the spring 225, thereby separating the valve element 231 from the valve seat 236. This allows the control fluid flowing in the inlet passage 234 to flow out from the outlet passage 235 via the valve chamber 233.
When a current to be applied to the coil 222 is changed at that time, the attractive force of the fixed core 223 will changes. This results in a change in stroke length of the movable core 224, thereby changing a distance between the valve seat 236 and the valve element 231. In this way, the valve opening can be adjusted to control a flow rate of the fluid allowed to flow out from the outlet passage 235.
However, in the aforementioned flow control valve 210, a linear stroke range of the movable core 224 (a proportional range between the voltage applied to the coil and the stroke length of the movable core) is small. The valve is therefore unsuitable for accurate flow control.
To increase the linear stroke range (the proportional range) of the movable core, therefore, a configuration that the fixed core and the movable core are designed to have tapered facing portions as shown in FIG. 28 has been practically used (Patent Document 2).
[Patent Document 1] JP63-180782 (1988)U
[Patent Document 2] JP7-19363 (1995)A